1. Field
One or more embodiments of the present invention relate to a method and sensor measuring skin impedance, and more particularly, to a method and sensor measuring skin impedance with improved accuracy using a measuring (M) electrode slightly recessed with respect to a reference (R) electrode and a current carrying (C) electrode causing the M electrode to makes contact with a user's skin after the R electrode and the C electrode.
2. Description of the Related Art
As more and more people become interested in beauty, interest in skin care has also increased. People also need to protect their skin from ultraviolet rays, which have become stronger due to the destruction of the ozone layer, and from various types of pollution. Therefore, a desire for a healthy skin relates not only to beauty-related products, but also to the growth of medical-related products.
Measuring skin moisture is a generally accepted method for self-diagnosing skin health. Moisture content in skin can be measured using a portable device, which may be implemented using skin impedance measurement techniques.
For example, U.S. Pat. No. 5,738,107 “Measurement of Moisture Content in Skin” discusses a device for determining relative skin moisture by measuring skin impedance. Here relative skin moisture is determined by measuring susceptance using an alternating current (AC) component of admittance, for three electrodes using a 50 KHz sinusoidal wave.
FIG. 1A illustrates a sensor and FIG. 1B illustrates a circuit for measuring skin impedance.
As illustrated in FIG. 1A, a sensor measuring skin impedance may include an R electrode, a C electrode, and an M electrode. When the sensor, having three electrodes, makes contact with a user's skin, a current begins to flow between the R electrode and the C electrode, through the skin. Here, the M electrode may measure impedance by measuring the current flowing between the R electrode and the C electrode.
In this case, noise may occur depending on how the electrodes make contact with the skin. Specifically, referring to a circuit illustrated in FIG. 1B, after power is supplied to an operational amplifier (Op Amp) 100, a voltage at pC is momentarily charged by an offset current to +Vcc or −Vcc. When the user's skin makes contact with the C and M electrodes, impedance for the skin becomes high due to a large amount of current that momentarily flows. However, when the R electrode makes contacts with the user's skin, the voltage on the C electrode returns to an original stable state.
Namely, when the R and C electrodes initially contact with the user's skin, subsequently the M electrode contacts with the user's skin, the impedance can be correctly measured. Conversely, when the R and M electrodes initially contact with the user's skin, subsequently the C electrode contacts with the user's skin, or when the C and M electrodes initially contact with the user's skin, subsequently the R electrode contacts with the user's skin, the impedance becomes abnormally high so that an error as illustrated in FIG. 2B occurs.
FIG. 2A is a graph illustrating when the impedance is normally measured, i.e. the R and C electrodes initially contact with the user's skin, subsequently the M electrode contacts with the user's skin. FIG. 2B is a graph illustrating when the impedance is abnormally measured, i.e. when the C and M electrodes initially contact with the user's skin, subsequently the R electrode contacts with the user's skin. In the above graphs, an x axis indicates times, i.e. seconds, and a y axis indicates impedance values.
As illustrated in FIG. 2A, when the impedance is correctly measured, a stable impedance value may be measured after approximately 1.3 seconds. Therefore, the impedance may be accurately measured since variations of the value are less for every subsequent measurement. Conversely, as illustrated in FIG. 2B, since an error has occurred at the beginning of the measurement, a stable impedance value may not be measured until after approximately 2.5 seconds to 4.1 seconds. Therefore, the impedance may not be accurately measured because, depending upon the measurement cycle, variations of the value are great in subsequent measurements.
To solve the above-described problems, one or more embodiments of the present invention disclose a method and sensor measuring skin impedance that can instantly and accurately measure the impedance of the user's skin with improved accuracy.